This came up on Stack Overflow recently and prompted a semi-official response:
This message will likely be removed in a future release. It's causing too much confusion than help and isn't entirely necessary. Yes it would be helpful for the developer log to let you know which addon is causing the problem, but the logistics of parsing backtraces to find that information proves to be difficult. Which brings us on to message #2:
As GDmac said, it's best to search your project for any calls to login_form() in the CP controller. Normally a file and line number would be provided in this kind of message, but as I said, parsing backtrace info doesn't always work.
We'll continue to try to make this better. The deprecation warnings have been working great for the most part, but occasionally messages like these pop up it can be difficult to know what to do about them.
